Proton exchange membrane (PEM) fuel cells are of interest for power generation from hydrogen fuel due to their high efficiency and near-zero emissions. They are typically based on an ion-conductive sulphonated fluoropolymer membrane such as Nafion® and operate in the 60-80° C. temperature range. Applications range from portable power to automobiles and on-site power-generation systems. Cost and durability concerns are the key barriers to their widespread use. Among the most expensive components in PEM fuel cells, and the dominant weight and volume portion of the fuel cell stack, are the bipolar plates. The bipolar plates serve to electrically connect the anode of one cell to the cathode of another in a stack to achieve a useful voltage. The bipolar plate also separates and distributes reactant and product streams through flow-field grooves on the faces of the plates.
Presently, graphite is the benchmark material for bipolar plates due to its electrically conductive and corrosion resistant in the highly aggressive anode and cathode PEM fuel cell where the acidic environment includes leached fluoride ion at a temperature of 60-80° C. Unfortunately, the brittleness and relatively high gas permeability of graphite necessitates the use of thick plates (>2-5 mm), which lowers the power density of the fuel cell stack. Machining of flow field groves into graphite plates is also expensive, making graphite impractical for most wide-scale commercial uses. Alternative bipolar plate materials include graphite/carbon-based composites, polymer-based composites with conductive graphite/carbon fillers, and metals. However, no cost effective material has definitively established itself as capable of meeting all of the properties that have been identified for the use of PEM fuel cells in automobile applications where high power densities required and are only easily achieved with bipolar plate thicknesses less than one millimeter.
Although graphite/carbon and polymer-based composites generally exhibit excellent corrosion resistance in PEM fuel cell environments, they have to be sealed to reduce gas permeability, have brittleness issues, and are very difficult to produce at the necessary thicknesses for automotive applications. The manufacture of graphite/carbon composites can also be costly, especially when measures are taken to mitigate their property shortcomings. Polymer-based composites are the current state of the art for bipolar plates and are available commercially. Cost targets appear achievable, but through-thickness conductivities are inadequate, being less than one third of the conductivities needed for automotive applications. Better conductivities appear to be achievable with very high loadings of conductive phase additions (graphite or carbon particles, fibers, nanotubes, etc.), although this can make the plates more difficult to manufacture. The high carbon loadings also tend to make the plates brittle, especially when making thin plates on the order of 0.5 mm to 1 mm thick.
Metallic alloys such as stainless steels would be ideal as bipolar plates because they are amenable to low-cost/high-volume manufacturing methods such as stamping, offer high thermal and electrical conductivities, have low gas permeability and excellent mechanical properties, and can be readily made in foil form of approximately 0.1 mm in thickness which permits high power densities. The primary limitations of metallic alloys are high contact resistance, borderline corrosion resistance, and cost.
Despite bulk electrical conductivities that are orders of magnitude greater the anticipated need, stainless steels generally exhibit interfacial contact resistance values that are too high by an order of magnitude for the goal in automobile applications due to the passive oxide layer present on stainless steels. This oxide layer is the source of the steels corrosion resistance. On exposure to the highly aggressive PEM fuel cell environments further growth of the oxide layer can increase the interfacial contact resistance. Dissolution of metallic ions from stainless steels can also occur under PEM fuel cell operating conditions. Sulphonated fluoropolymer membranes are very sensitive to poisoning by metallic ions, and the fuel cell performance can be significantly degraded at contamination levels of the order of 10-100 ppm metallic ion. For automotive applications, the high interfacial contact resistance and borderline corrosion resistance of stainless-steel are not acceptable with conventional fuel cell designs. Other metallic materials have also been investigated as bipolar plate materials, particularly Ni—Cr, titanium, and refractory metals such as niobium and tantalum. However, the cost of these materials is generally in excess of that required for automotive applications, and interfacial contact resistance values and/or corrosion resistance are still borderline with respect to the goals.
To meet bipolar plate targets for automotive applications, metallic bipolar plates will require conductive corrosion-resistant coatings or surface treatments. Unfortunately, coatings for metallic bipolar plates have thus far not proven sufficiently viable due to local areas of inadequate surface coverage such as pin-hole defects, which result in local corrosion and metallic ion contamination of the membrane. Due to the sensitivity of the sulphonated fluoropolymer membranes to poisoning by metallic ions and the aggressiveness of the PEM fuel cell operating environment, bipolar plates require a fully dense, essentially defect-free protective coating. This is especially true for low-cost but less corrosion-resistant metal substrates such as low-alloy steels or aluminum, which can be rapidly attacked in PEM fuel cell environments. Methods to mitigate the presence of pin-hole defects (i.e., the use of interlayers) are being pursued, but can significantly increase costs. Difficulties are also encountered in obtaining full coverage of complex flow field corner and edge geometries. Hence, the need remains to modify a stainless-steel surface in a cost effective manner that is essentially defect free, corrosion resistant and does not have a prohibitive interfacial contact resistance such that metal alloys can be used in PEM fuel cells for automotive applications.
As the foreseeable costs of nickel do not appear to encourage the use of Ni—Cr based alloys for bipolar plates in an automotive application, the use of ferritic type stainless-steel alloys where nickel content is very low or non-existent is desirable. The use of nickel containing austenitic type stainless-steel, though more expensive than a ferritic type stainless-steel, would be desirable over a Ni—Cr based alloy.